Gas-liquid chromatography

From Academic Kids

Gas-liquid chromatography (GLC), or simply gas chromatography (GC) is a type of chromatography in which the mobile phase is a carrier gas, usually an inert gas such as helium or nitrogen, and the stationary phase is a microscopic layer of liquid on an inert solid support. The stationary phase lines the inside of a very long very thin tube known as a column.

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History

Chromatography dates to 1903 in the work the Russian scientist, Mikhail Semenovich Tswett. His work went largely unrecognized probably because he published it only in Russian, and because of the fact he was widely known as a homosexual. German graduate student Fritz Prior developed solid state Gas Chromatography in 1947. Archer John Porter Martin, who was awarded the Nobel Prize for his work in developing liquid-liquid (1941) and paper (1944) chromatography, laid the foundation for the development of gas chromatography and later produced liquid-gas chromatography (1950).

GC analysis

A gas chromatograph is a chemical analysis instrument for separating and identifying chemicals in a sample. A gas chromatograph uses a thin capillary fiber known as the column, through which different chemicals pass at different rates depending on various chemical and physical properties. As the chemicals exit the end of the column, they are detected and identified electronically. The function of the column is to separate and concentrate different components in order to maximize the detection signal.


In a GC analysis, a known volume of gaseous or liquid analyte is injected into the entrance of the column, usually using a microsyringe. Although the carrier gas sweeps the analyte molecules through the column, this motion is inhibited by the adsorption of the analyte molecules either onto the column walls or onto packing materials in the column. The rate at which the molecules progress along the column depends on the strength of adsorption, which in turn depends on the type of molecule and on the column materials. Since each type of molecule has a different rate of progression, the various components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times. A detector is used to monitor the outlet stream from the column; thus, the time at which each component reaches the outlet and the amount of that component can be determined. Generally, substances are identified by the order in which they emerge from the column and by the residence time of the analyte in the column.

Two types of columns are used in GC:

  • Packed columns contain a finely divided, inert, solid support material (eg. diatomaceous earth) coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed. Thus numerous columns are available that are designed to separate specific types of compounds. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm. The outer tubing is usually made of stainless steel or glass.
  • Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters. The column walls are coated with the active materials. Most capillary columns are made of fused-silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil.

Because molecular adsorption and the rate of progression along the column depend on the temperature, the column temperature is carefully controlled to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times. For some cases temperature is ramped either continuously or in steps to provide the desired separation.

A number of detectors are used in gas chromatography. The most common one is the thermal conductivity detector (TCD), which monitors changes in the thermal conductivity of the effluent. The main advantage of the TCD is that it can detect any substance (except the carrier gas). Some of the other detectors are only sensitive to specific types of substances. Other detectors include the flame ionization detector (FID), electron capture detector (ECD), flame photometric detector (FPD), photo-ionization detector (PID), and Hall electrolytic conductivity detector.

One example of the use of gas chromatography is in the study of the selectivity of Fischer-Tropsch synthesis catalysts. The outlet from this process contains a number of light gases including N2, H2, CO, CO2, H2, CH4, and Ar, as well as heavier parafinic and olefinic hydrocarbons (C2-C40). In a typical experiment, a packed column is used to separate the light gases, which are then detected with a TCD. The hydrocarbons are separated using a capillary column and detected with an FID.

Manufacturers of gas chromatographs

See also

Gas_chromatography-Mass_spectrometryde:Gaschromatografie ja:ガスクロマトグラフィー nl:Gaschromatografie ru:Газовая хроматография

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